Sediment Management Plan Rehoboth Bay STAC Meeting November 16, 2007
Goals of the Study Improve planning for future dredging needs Reduce the dependency on dredging Study Tasks Environmental data Historical dredge records Sediment characteristics Hydrodynamics Sediment deposition and erosion characteristics Developing ways to strategize a long term sediment management plan.
M&N s Scope of Work 1. Existing Conditions Assessment 2. Review of Historical Dredging and Disposal 3. Hydrodynamic Modeling 4. Shoaling Estimates 5. Sediment Management Alternatives
Task 1. Existing Conditions
Study Area Rehoboth Bay is a shallow bar-built estuary with depths less than 6 to 7 ft (MLLW). Rehoboth Bay has tidal exchange through the southern link to Indian River Bay and through the Lewes Rehoboth Canal. It also receives freshwater inflow from the many tidal creeks. Six primary channel dredging projects within Rehoboth Bay: Lewes-Rehoboth Canal Love Creek Herring Creek Guinea Creek Wilson Creek Massey s Ditch
Shoreline Change
Western Shoreline
Herring Creek
Lewes-Rehoboth Canal
Task 2. Historical Dredging and Disposal
USACE Dredging In recent years USACE has only maintained the L-R Canal north of the Savannah Ave Bridge Fiscal Year 1990 1994 1998 2001 2004 Job Name L & R CANAL - ROOSEV.INLET ROOSEVELT, MURDERKILL, MISPILLION IWW, REH. BAY TO DEL. BAY IWW, REH. BAY TO DEL. BAY ROOSEVELT INLET,DE. Dredge Type Pipeline Pipeline Pipeline Pipeline Pipeline Disposal Type Beach Nourishment Mixed (More than one type) Beach Nourishment Beach Nourishment and Upland Beach Nourishment Arrival Date Departure Date Volume Removed (CY) not avail not avail not avail 10/23/1994 12/15/1994 94,873 10/28/1998 11/30/1998 29,557 10/22/2001 3/1/2002 46,102 8/25/2004 10/30/2004 180,745
DNREC Dredging since 1970 Decade Inland Bays Waterway Dredging (x1,000 cy) Other (x1,000 cy) Total (x1,000 cy) 1970's 363 437 853 1980's 309 1589 1,898 1990's 244 1,326 1,570 2000-109 116 225 TOTAL 1,025 3,468 4,546
DNREC Dredging in Inland Bays WATERWAY YEAR TOTAL Initial Maint. Lewes-Rehoboth Canal 1989 TO 1991 40 40 Love Creek 1970 TO 1971 115 115 Herring Creek 1978 TO 1983 85 85 Guinea Creek 1977 75 75 Wilson Creek 1983 27 27 Cozy Cove 1978 TO 1979 18 18 Massey s Ditch 1987, 1990 TO 1991 62 62 Indian River Bay 91 TO 1992, 1995 TO 1996, 2001 147 147 Pepper Creek 1986 TO 1988, 1991 TO 1993 220 220 Vines Creek 1993 7 7 White Creek 970 TO 1971, 1997, 1999 TO 200 195 135 60 Assawoman Canal 2006 34 34 Grand Total Navigation 1,025 682 343 Average Annual Maintenance 9.3
Task 3. Hydrodynamic Modeling
DELFT 3D Modeling System An integrated surface water modeling system developed by WL Delft Hydraulics Two and three-dimensional flow, waves, water quality, ecology, sediment transport and bottom morphology
Model Grid
Bathymetry Data Date Coverage Source 2004 IR Inlet and Surrounding areas USACE 2004 1963,1970,1977, 1984 2004 2004 1998 2000 2005 Inland Bays Offshore Love Creek Herring Creek Guinea Creek Bald Eagle Creek Roosevelt Inlet (Lewes Rehoboth Canal) DNREC NGDC (GEODAS) DNREC DNREC DNREC DNREC USACE
Model Bathymetry
Model Bathymetry
Offshore Boundary Tidal constituents were extracted at the north and south ends of the boundary from the Advanced CIRCulation (ADCIRC) model EastCoast2001 Residuals from three nearby NOAA stations were compared with those at Indian River Inlet. Extracted residuals were added to the harmonic components of the boundary
Additional Forcing River Discharge was found to have negligible effects on hydrodynamic conditions with the exception of Indian River. Wind forcing has the greatest impact in Rehoboth Bay. The closest available source for the tested time periods was at Georgetown-Sussex Airport
Calibration
Harmonic Calibration 1 Elevation (ft) 0.5 0 0.5 1 05/01/02 05/08/02 05/15/02 05/22/02 05/29/02 Station Correlation RMS error (ft) Percent error Indian River Inlet (Coast Guard Station) Indian River Bay (Rosedale Beach) Rehoboth Bay (Dewey Beach) 0.99 0.20 5.8% 0.99 0.13 4.3% 0.98 0.08 5.3%
Water Level Calibration 4 Elevation (ft, NGVD29) 3 2 1 0 1 2 05/01/02 05/08/02 05/15/02 05/22/02 05/29/02 Station Correlation RMS error (ft) Percent error IRI (Coast Guard 0.98 0.22 4.5% Station) Indian River Bay 0.98 0.25 5.2% (Rosedale Beach) Rehoboth Bay (Dewey Beach) 0.95 0.27 8.4%
Water Levels
Water Levels
Maximum velocities
Task 4. Shoaling Calculations
Lewes-Rehoboth Canal Lewes Reach 5,000 to 10,000 cy/year (excluding RI inlet) >90% sand 2-4 yr cycle Most sedimentation occurs in the turning basin and bend immediately north of this area
Lewes-Rehoboth Canal Henlopen Acres Reach 1,000 to 2,000 cy/yr, 2-4 yr cycle. Less than 30% sand
Lewes-Rehoboth Canal Rehoboth Bridge 2,000 cy/yr, 2-4 yr cycle. More than 90% sand
Lewes-Rehoboth Canal Rehoboth Bay Jetties 4,000 cy/yr, 1-2 yr cycle. 50% sand 50% silt/clay Total initial volume for L-R Canal: 40,000 cy
Love Creek 3-5,000 cy/yr. Mostly silt/clay Total initial volume for -4 ft MLW channel: 60,000 cy
Herring and Guinea Creek 4-8,000 cy/yr for a channel at -4ft MLW. Mostly silt/clay
Massey s Ditch Problem Areas: A. North of Bluff Point B. Pullover C. Massey s Ditch D. Middle Island shoal E. Channel from Big Ditch to Little Ditch
Analytical Sedimentation Model Analytical methodology originally developed by Eysink and Vermaas (1983) which accounts for: Channel dimensions Natural water depth Settling velocity of sediment Estimated wave conditions Hydrodynamic model was modified to represent post-dredge conditions.
A. Bluff Point USACE Surveys 2001 2003 2004
A. Bluff Point Maximum Velocity (ft/s)
B. Pullover USACE Surveys 2001 2003
B. Pullover Velocity (ft/s)
C. Massey s Landing USACE Surveys 2001 2003
C. Massey s Landing Velocity (ft/s)
D. Middle Island USACE Surveys 2001 2004
D. Middle Island Velocity (ft/s)
E. Big Ditch to Little Ditch Channel
E. Big Ditch to Little Ditch Channel Velocity (ft/s)
Massey s Ditch Shoaling Rates Location A: Bluff Point B: Pullover C: Massey s Landing D: Middle Island E: Big Ditch to Little Ditch Channel Shoaling Rate ft/yr 2.2 2.7 2.3 3.4 (2) 1.8 Shoaling Rate cy/yr 8,000 3,000 3,000 8,000 2,000 Dredging frequency (1) (1) Approximate Dredging frequency required to maintain a channel at -6 ft MLW (2) This rate is not across the entire width of the channel, only at the leading edge of the shoal 2-4 1-2 1-2 2-4 2-4
Task 5. Sediment Management Alternatives
Shoaling Reduction Measures KSO: Keep Sediment Out Dikes Sills Harbor entrance modifications Sedimentation basins/traps KSM: Keep Sediment Moving Flow training structures Flow augmentation
Volume Reduction Measures Mechanical Dewatering: Belt Press Dredging Garrisons Lake near Smyrna (8,000 cy) Beneficial Reuse Measures Habitat Restoration & Development Wetlands (2,000 acres lost over last century) Thin Layer Spreading Aquatic Habitat Filling anoxic holes & dead en canals Island Restoration Beach Restoration
Camp Arrowhead
Lewes-Rehoboth Canal
Lewes-Rehoboth Canal Projected Maintenance Dredging Volumes & Frequency Estimates are based on authorized project dimensions, including a channel depth of - 6 ft MLW. Lewes Reach: 5,000 to 10,000 cy/yr (RI not included). 2-4 year cycle. > 90% sand. Cape Henlopen State Park Reach: no dredging required. Henlopen Acres Reach: 1,000 to 2,000 cy/yr. 2-4 year cycle. < 30% sand. Rehoboth Beach Reach: 2,000 cy/yr. 2-4 year cycle. > 90% sand. Thompson Island Reach: 4,000 cy/yr. 1-2 year cycle. 50% sand and 50% silt/clay. Initial dredging volume likely to exceed 40,000 cy. Current Management & Issues Canal north of Savannah Ave Bridge last dredged by USACE in 2002. Disposal to Lewes CDF. The State dredged between Lewes and Henlopen Acres Marina in 1989-91. Disposal to two CDFs: City of Lewes and Mr. Andersen s Property across from Henlopen Acres. From Rehoboth Beach to the confluence with Rehoboth Bay the canal has not been dredged since the 1960 s. The State planned to dredge in this area but did not for lack of practical disposal options. Very limited federal funding available. USACE dredging unlikely south of RI
Potential SM Strategies Disposal Continued disposal to City of Lewes CDF for areas north Savannah Bridge KSM Removal of RR bridge abutment > Smoother flow conditions and increased exchange between Lewes and Rehoboth. Flow training wall at Henlopen Acres Marina Restoration of the Thompson Island Jetties No practical KSM elsewhere KSO Bank stabilization in Rehoboth.
Potential SM Strategies Beneficial Reuse Beach restoration at Lewes Marsh restoration on Thompson Island. Will likely require structural stabilization Thin-layer spraying Filling of anoxic holes in Bald Eagle Creek
Savannah Bridge
Henlopen Acres Marina
Thompson Island Marsh Restoration Thompson Island Terminal Structure Fill with dredged sediment Shoreline Protection Structure Existing Jetties Land loss since 1845 R e h o b o t h B a y
Low relief protection wall
Holt s Landing
Love Creek
Potential SM Strategies Disposal Exiting Robinson Landing site. Limited Capacity. 2 miles to the Rt 24 bridge KSM Channel marking: Boat traffic effects. KSO Not possible Beneficial Reuse Island restoration: Marsh and Big Piney, 5-10 acres. 1 mile to 3 miles. Will require containment and protection structures. Mainland marsh restoration (e.g., south of Joy Beach)
R E H O B O T H B A Y Horse Island Marsh Island Shoreline Protection Structure Fill with dredged sediment Big Piney Island Shoreline Protection Structure Unprotected containment Fill with dredged sediment
Herring & Guinea Creek
Potential SM Strategies Disposal Two areas possibly available: Shawns Hideaway & Winding Creek Village. Limited capacity KSM Channel marking. Expanded program into the Prongs and Guinea Creek KSO Not possible Beneficial Reuse Island restoration: Island in the Narrows. Mainland marsh restoration in the creek and along Long Neck Restoration at Joseph Lee Creek Filling of dead-end canals and ponds. Mouth of Joseph Lee Creek (Lingo Landing). Bayview Lane in Lingo Cove Thin-layer spreading
Wolfpit Marsh Angola Landing Herring Creek Guinea Creek Shoreline Protection Structure Unprotected containment Fill with dredged sediment Rehoboth Bay Wilson Marsh
Potential Marsh Restoration Area at Joseph Lee Creek
Lingo Cove
Massey s Ditch
Potential SM Strategies Disposal No disposal sites available in the immediate vicinity Bottom Hills Drain. 2 to 3 miles transport distance. 320,000 cy capacity. KSM Flow training features. Possibly in combination with marsh restoration KSO Not possible Beneficial Reuse Aquatic habitat restoration at BHD Marsh restoration Beach restoration: bay and ocean shorelines
Flow Training and fill containment structure Shoreline Protection Structure Fill with dredged sediment (typ.) Low relief containment structure (sill)
Flow Training and fill containment structure Shoreline Protection Structure Fill with dredged sediment (typ.) Rock groins
Angola Landing Restoration
Massey s to Middle Island D s y' se as M h itc Flow Training Wall Middle Island Big Ditch Little Ditch
THANK YOU!